dvdnav: also white-list *.img files (fixes #4951)

(cherry picked from commit 00eb305f2afebf28dd649d507aca4901d0ad1e8a)

modules/access/dvdnav.c

@@ -1467,8 +1467,11 @@ static int ProbeDVD( demux_t *p_demux, char *psz_name )
     }
 
     /* Match extension as the anchor exhibits too many false positives */
-    const size_t len = strlen( psz_name );
-    if( len < 4 || strcasecmp( psz_name + len - 4, ".iso" ) )
+    const char *ext = strrchr( psz_name, '.' );
+    if( ext == NULL )
+        goto bailout;
+    ext++;
+    if( strcasecmp( ext, "iso" ) && strcasecmp( ext, "img" ) )
         goto bailout;
 
     /* Try to find the anchor (2 bytes at LBA 256) */